1. Field of the Invention
The present invention relates to a display device which has a power source circuit, and more particularly to a display device which has a charge pump control circuit formed of thin film transistors.
2. Description of the Related Art
In recent years, with the advance of the communication technology, mobile phones have been widely used. In future, transmission of moving images and transmission of a larger volume of information are expected. On the other hand, through reduction in weight of personal computers, those adapted for mobile communication have been produced. Information terminals called PDA originated in electronic notebooks have also been produced in large quantities and widely used. In addition, with the development of display devices, the majority of portable information devices are equipped with flat panel displays.
Conventionally, polycrystalline semiconductor films were formed at 1000° C. or more. However, in recent years, the films are formed at a low temperature of approximately 500° C. at highest. With the low-temperature polycrystalline semiconductor TFTs (Thin Film Transistor), manufacturing of an active matrix display device has been promoted. Such an active matrix display device has advantages in that, in addition to a pixel, a signal line driving circuit can be integrally formed around a pixel portion. Thus, since it is possible to realize downsizing and high definition of a display device, the display device is expected to be more widely used in future.
However, although a circuit to write a video signal to a pixel was incorporated in an original display device formed by using low-temperature polycrystalline semiconductor TFTs, a power source circuit or the like was not incorporated and it was provided as the externally attached part.
Generally, a lithium ion battery is used as a power source for portable equipment such as portable information equipment. The lithium ion battery normally outputs direct current voltages of approximately 3.6 V and is widely used for the advantages of a long life, a high-speed charge, a good retention characteristic and safety. However, to drive a material such as liquid crystal or organic EL (electro luminescence) used for a display device, the voltage of 3.6V is insufficient and voltage of 10V to 18V is required.
For the above reason, a display device as shown in FIG. 2 in which a charge pump circuit is configured on a substrate to supply voltages required for driving was developed. FIG. 2 is an outline view of the periphery of a display device of portable information equipment having a conventional charge pump. A pixel portion 204, a source signal line driving circuit 202, a gate signal line driving circuit 203, a switching element 205 are integrally formed on a substrate 201. Capacitors 207 and 208 are loaded on an FPC (Flexible Printed Circuit) 206. A clock generator 209 is provided outside the substrate 201. Note that the charge pump comprises the switching element 205 and capacitors 207 and 208.
FIG. 3 shows a conventional charge pump circuit. Here, switching elements are connected to drains/gates of N-type TFTs and used as diodes. The operation will be hereinafter described. First, a voltage of a power source 301 is applied to a capacitor 304 via a switching element 302. In the case where the voltage of the power source 301 is referred to as VDD and the voltage of the switching element is referred to as VF, a voltage of VDD-VF is applied to the both ends of the capacitor 304 when the output of a clock generator 307 is Lo. Next, when the output of the clock generator is Hi, a charge of the capacitor is applied to a load 306 and a capacitor 305 via a switching element 303. When the current flowing in the load is small enough, the charge of the capacitor 304 is retained, thereby a voltage of 2VDD-2VF generates at the both ends of the capacitor 305. In case of VDD>>VF, a voltage of nearly second times as high as VDD is generated in the load. Accordingly, a higher voltage than the original one can be obtained by using a charge pump. This is shown in FIGS. 4A and 4B.